Dielectric block bandpass filters, for example as in U.S. Pat. No. 4,431,977, "Ceramic Bandpass Filter", are commonly used as signal filters in communication systems, for example as in a conventional radio, transceiver, or radiotelephone. Conventonal dieletric filters off advantages in both physical and electrical performance which make them ideally suited for use in mobile and portable radio transceivers. Multi-resonator dielectric filters, as depicted in FIG. 1, typically comprise a plurality of quarter-wavelength transmission line resonators, constructed by making through-holes in the dielectric material, and plating these holes with a conductive material. In such a configuration, reactive coupling between adjacent resonators can be controlled by the physical dimensions of each resonator and by the orientation of each resonator with respect to the other resonators.
It is also commonplace to use diodes for electronically switching components in applications such as voltage controlled oscillators (hereinafter "VCO"), duplexers, and other areas where high speed switching is required. Voltage variable capacitors are also used for tuning in applications such as electronically tuned helical resonators, for example in U.S. Pat. No. 4,459,571, "Varactor-tuned Helical Resonator Filter".
The tuning of resonator loads is not a new idea, and such a resonant load may be a VCO, as in the aforementioned U.S. patent application No. 538,874. Unfortunately, such a configuration relies on relatively high voltage levels generated by the VCO in order to cause self-rectification in the diode switch, resulting in a reverse bias condition. In typical filter applications, there are not sufficient voltage levels to guarantee reverse biasing, hence the switch could never be in an `off` state using this technique. Regarding the use of voltage variable capacitors, these devices typically offer very poor temperature stability, thereby requiring additional components for temperature compensation. The cost of adding these components can be significant when compared to the total cost of the filter. U.S. Pat. No. 4,800,348, "Adjustable Electronic Filter and Method of Tuning Same", shows the use of tuning networks which are mechanically trimmed to affect a change in the center frequency of the filter, but this process is typically a time consuming and costly process.
In short, shown solutions to the adjustment of the center frequency of bandpass filters used in communication systems, especially for miniaturized, dielectrically loaded resonator filters, are inadequate when an agile center frequency is desired. It would be advantageous to have one filter which could be electronically configured for use in multiple frequency bands, such as the case in a conventional cellular radiotelephone operating domestically at one frequency and at another frequency when operating in a foreign country. Clearly, there is a need for a temperature stable, electronically selectable center frequency bandpass filter which is not constrained by the aforementioned shortcomings.